Random access PPDU for WLAN systems

In wireless communications for multi-users, an access point may transmit a first trigger frame to one or more stations. The first trigger frame may be for scheduling a first uplink multi-user transmission and may indicate a plurality of resources for indicating existence of data to be sent to the access point. The station(s) may process the first trigger frame received from the access point. In response to the first trigger frame, the station(s) may generate and transmit a respective first uplink frame to the access point. In some cases, the first uplink frame(s) from the station(s) does not have a data field and does have a signal indicating existence of data to be sent from the station(s) to the access point. The signal may be sent using a resource(s) of the plurality of resources. Other methods, apparatus, and computer-readable media are also disclosed.